Vertical floating gate transistor

ABSTRACT

A floating gate transistor has been described that includes source and drain regions that are fabricated on different horizontal planes. A floating gate and a control gate are fabricated vertically to control current conducted through the transistor. The control gate is coupled to a word line that is formed with the control gates and extends in a common horizontal direction.

FIELD OF THE INVENTION

The present invention relates generally to non-volatile memory cells andin particular the present invention relates to flash memory cells.

BACKGROUND OF THE INVENTION

Memory devices are available in a variety of styles and sizes. Somememory devices are volatile in nature and cannot retain data without anactive power supply. A typical volatile memory is a DRAM which includesmemory cells formed as capacitors. A charge, or lack of charge, on thecapacitors indicate a binary state of data stored in the memory cell.Dynamic memory devices require more effort to retain data thannon-volatile memories, but are typically faster to read and write.

Non-volatile memory devices are also available in differentconfigurations. For example, floating gate memory devices arenon-volatile memories that use floating gate transistors to store data.The data is written to the memory cells by changing a threshold voltageof the transistor and is retained when the power is removed. Thetransistors can be erased to restore the threshold voltage of thetransistor. The memory may be arranged in erase blocks where all of thememory cells in an erase block are erased at one time. Thesenon-volatile memory devices are commonly referred to as flash memories.

The non-volatile memory cells are fabricated as floating gate memorycells and include a source region and a drain region that is laterallyspaced apart from the source region to form an intermediate channelregion. The source and drain regions are formed in a common horizontalplane of a silicon substrate. A floating gate, typically made of dopedpolysilicon, is disposed over the channel region and is electricallyisolated from the other cell elements by oxide. For example, gate oxidecan be formed between the floating gate and the channel region. Acontrol gate is located over the floating gate and is can also made ofdoped polysilicon. The control gate is electrically separated from thefloating gate by another dielectric layer. Thus, the floating gate is“floating” in dielectric so that it is insulated from both the channeland the control gate.

As semiconductor devices get smaller in size, designers are faced withproblems associated with the production of memory cells that consume asmall enough amount of surface area to meet design criteria, yetmaintain sufficient performance in spite of this smaller size.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art foran improved non-volatile memory cell.

SUMMARY OF THE INVENTION

The above-mentioned problems with non-volatile memory cells and otherproblems are addressed by the present invention and will be understoodby reading and studying the following specification.

In one embodiment, a pair of floating gate transistors comprises apillar of silicon vertically extending from a substrate, an implanteddrain region located in a top region of the pillar; and a pair of sourceregions implanted in the substrate. The pair of source regions arelocated on opposite sides of the pillar. First and second floating gatesare located on opposite sides of the pillar and insulated from thepillar by gate oxide. First and second control gates are located onopposite sides of the pillar and insulated from the first and secondfloating gates.

In another embodiment, a non-volatile memory array comprises a pillar ofsilicon vertically extending from a substrate, an implanted drain regionlocated in a top region of the pillar; and a pair of source regionsimplanted in the substrate. The pair of source regions are located onopposite sides of the pillar. First and second floating gates arelocated on opposite sides of the pillar and insulated from the pillar bygate oxide. First and second control gates are located on opposite sidesof the pillar and insulated from the first and second floating gates.The first and second control gates are fabricated as parallelpolysilicon rails, and the pair of source regions are fabricated asparallel source implant rails generally extending under the first andsecond control gates. A drain contact vertically extends from the topregion of the pillar, and first and second source contacts verticallyextend from the pair of source regions. A source conductor is coupled toa top of the first and second source contacts. Finally, a drainconductor is coupled to a top of the drain contact. The source and drainconductors extend in a horizontal direction generally perpendicular tothe control gates.

A method of fabricating non-volatile memory cells comprises forming apillar of silicon vertically extending above a substrate, implanting adrain region in a top of the pillar, and implanting first and secondsource regions in the substrate and adjacent to the pillar. The firstand second source regions are located on opposite sides of the pillar.The method further comprises depositing a gate oxide on at leastvertical sides of the pillar facing the first and second source regions,forming floating gates adjacent to and on opposite sides of the pillar,and forming first and second control gates insulated from the floatinggates and located on opposite sides of the pillar.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of floating gate transistors of an embodimentof the present invention;

FIG. 2 is a plan view of a portion of a memory array of an embodiment ofthe present invention;

FIG. 3 is a cross-section of the memory array of FIG. 2;

FIGS. 4-15 illustrate a fabrication process of an embodiment of thepresent invention; and

FIG. 16 is a block diagram of a memory of an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following detailed description of the invention, reference ismade to the accompanying drawings that form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention.

To aid in the interpretation of the detailed description and claims thatfollow, the term “semiconductor substrate” is defined to mean anyconstruction comprising semiconductive material, including, but notlimited to, bulk semiconductive materials such as a semiconductor wafer(either alone or in assemblies comprising other materials thereon) andsemiconductive material layers (either alone or in assemblies comprisingother materials). The term “substrate” refers to any supportingstructure, including, but not limited to, the semiconductor substratesdescribed above. The term substrate is also used to refer tosemiconductor structures during processing, and may include other layersthat have been fabricated thereupon. Both wafer and substrate includedoped and undoped semiconductors, epitaxial semiconductor layerssupported by a base semiconductor or insulator, as well as othersemiconductor structures well known to one skilled in the art.

In addition, as the structures formed by embodiments in accordance withthe present invention are described herein, common semiconductorterminology such as N-type, P-type, N+ and P+ will be employed todescribe the type of conductivity doping used for the various structuresor regions being described. The specific levels of doping are notbelieved to be germane to embodiments of the present invention; thus, itwill be understood that while specific dopant species and concentrationsare not mentioned, an appropriate dopant species with an appropriateconcentration to its purpose, is employed.

The term conductor is understood to also include semiconductors, and theterm insulator is defined to include any material that is lesselectrically conductive than the materials referred to as conductors.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims, along with the full scope of equivalents towhich such claims are entitled.

Finally, it will be understood that the number, relative size andspacing of the structures depicted in the accompanying figures areexemplary only, and thus were selected for ease of explanation andunderstanding. Therefore such representations are not indicative of theactual number or relative size and spacing of an operative embodiment inaccordance with the present invention.

Non-volatile memory cells, as explained above, can be formed as floatinggate transistors. While the area of the transistors can be reduced, thecoupling ratio between the floating gate and the control gate is alsoreduced. Generally, as the gate-coupling ratio between the floating gateand the control gate decreases, the work voltage necessary to operatethe memory transistor increases. As a consequence, the operational speedand efficiency of the flash memory decrease tremendously.

Some methods for increasing the gate-coupling ratio include: increasingthe overlapped area between the floating and the control gate, reducingthe thickness of the dielectric layer between the floating gate and thecontrol gate, and increasing the dielectric constant (k) of thedielectric layer between the floating gate and the control gate.Generally, to achieve an increase in the overlapped area between thefloating and control gates and thus increase the gate-coupling ratio,the size of the floating gate has to be increased. However, this is notdesirable for the demands of today's highly-integrated technologies.

Referring to FIG. 1, a cross-section view of two floating gatetransistors 100 according to one embodiment of the present invention isprovided. The transistors includes a source 102, drain 104, floatinggate 106 and a control gate 108. Unlike prior transistors, the sourceand drain regions of the transistors of the present invention are notlocated in a common horizontal plane of the memory device substrate. Asexplained below, the source and drain regions are fabricated indifferent planes such that a channel 110 of the transistor extends in avertical direction. The floating gate and control gate also extendvertically into the substrate to control the transistor.

As shown in FIGS. 2 and 3, multiple floating gate memory celltransistors are fabricated in a substrate 200. FIG. 2 is a plan view ofa portion of a memory array and FIG. 3 is a cross-section view of thearray. The transistors share a common diffused source region 202, andthe drain regions 204 are fabricated in a plane above the source region.Two adjacent transistors have independent floating gates 206 and 208fabricated in trenches. A control gate 210 is fabricated between memorycells and is coupled to a word line 212 that runs parallel to the lengthof the control gates. Described in an alternate way, a pillar of siliconhas two transistors fabricated therein. A top of the pillar has a commondrain and two source regions are formed toward the bottom of the pillar.Floating gates are formed on opposite sides of the pillar. Each floatinggate has a separate control gate. The sources and control gates extendin general parallel directions.

The features of one embodiment of the present invention can be describedin greater detail with reference to a method of fabricating thetransistors. The method is described in sufficient detail to provide anunderstanding of one method to form transistors of the presentinvention. It will be understood by those skilled in the art that allprocess steps are not described in detail herein, and that extra stepsor modifications of the steps may be needed depending upon theintegrated circuit design and manufacturing equipment.

FIG. 4 is a perspective cross-section view of a semiconductor substrate.The substrate has a layer of oxide (not shown) and a layer of Nitride220 formed on the top surface of the substrate. The Nitride layer isthen patterned and etched in the X direction to form shallow trenches222 into the substrate. A layer of thin oxide is then grown and thetrenches are filled with oxide 224. A chemical-mechanical planarization(CMP) operation is then performed to remove oxide above the Nitridelayer. As such, the trenches are replaced with oxide isolation rowsextending in the X direction; see the side view illustrated in FIG. 5.

Referring to FIGS. 6 and 7, the top surface is then patterned and etchedto form trenches running perpendicular to the oxide isolation rows. As aresult, pillars 240 of substrate (active) material and oxide isolation224 are formed. Floating gates and control gates will be formed in thetrenches, as described below. As such, two transistors are formed usingone active pillar. Each transistor has a separate control gate formed onopposite sides of the pillar.

A tunnel oxide layer 242 is grown over the pillars and a layer ofpolysilicon 244 is deposited over the oxide, see FIG. 8. This layer ofpolysilicon is used to form the floating gates. Optionally, a layer ofsilicide can be combined with the polysilicon to form the floatinggates. Referring to FIGS. 9 and 10, the polysilicon is patterned andetched to define the vertical edges of the floating gates 246. Thehorizontal edges of the floating gates are formed during the etch, orusing a second subsequent etch operation. An implant operation isperformed to form the source regions 250 in the bottom of the trenches230. As such, the sources are located vertically beneath the controlgate locations.

Referring to FIGS. 11-13, an oxide-nitride-oxide (ONO) thin filmdielectric 252 is deposited to insulate the second floating gates and apolysilicon layer 254 is deposited. A layer of tungsten 256 is depositedin the groove 258 of the polysilicon. A CMP operation is then performedto remove portions of the tungsten and second polysilicon to form thecontrol gate 260. The cross-section of the control gate, see FIG. 12,resembles a U-shape polysilicon channel, or rail, with a center oftungsten.

In subsequent processes, the nitride material 220 is stripped and thedrain regions 262 are formed through an implant operation at the top ofthe pillars. An insulating layer 270, such as boro-phospho-silicateglass (BPSG), is deposited and processed to form contact openings. Thecontact openings are filled with a conductor or semiconductor to formcontacts 274. A metal layer is then formed and processed to create bitlines 276 coupled to the drain regions via the contacts 274, see FIG.14. FIG. 15 illustrates the source regions 250 and source contacts 280that provide electrical contact between the source and a source line282.

FIG. 16 is a functional block diagram of a memory device 400, of oneembodiment of the present invention, which is coupled to a processor410. The memory device 400 and the processor 410 may form part of anelectronic system 420. The memory device 400 has been simplified tofocus on features of the memory that are helpful in understanding thepresent invention. The memory device includes an array of memory cells430. The memory cells are non-volatile floating-gate memory cells withvertical floating gates. The memory array 430 is arranged in banks ofrows and columns.

An address buffer circuit 440 is provided to latch address signalsprovided on address input connections AO-Ax 442. Address signals arereceived and decoded by row decoder 444 and a column decoder 446 toaccess the memory array 430. It will be appreciated by those skilled inthe art, with the benefit of the present description, that the number ofaddress input connections depend upon the density and architecture ofthe memory array. That is, the number of addresses increases with bothincreased memory cell counts and increased bank and block counts.

The memory device reads data in the array 430 by sensing voltage orcurrent changes in the memory array columns using sense/latch circuitry450. The sense/latch circuitry, in one embodiment, is coupled to readand latch a row of data from the memory array. Data input and outputbuffer circuitry 460 is included for bi-directional data communicationover a plurality of data (DQ) connections 462 with the processor 410.Write circuitry 455 is provided to write data to the memory array.

Command control circuit 470 decodes signals provided on controlconnections 472 from the processor 410. These signals are used tocontrol the operations on the memory array 430, including data read,data write, and erase operations. The flash memory device has beensimplified to facilitate a basic understanding of the features of thememory. A more detailed understanding of internal circuitry andfunctions of flash memories are known to those skilled in the art.

CONCLUSION

A floating gate transistor has been described that includes source anddrain regions that are fabricated on different horizontal planes. Afloating gate and a control gate are fabricated vertically to controlcurrent conducted through the transistor. The control gate is coupled toa word line that is formed with the control gates and extends in acommon horizontal direction.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiment shown. This applicationis intended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. A pair of floating gate transistors comprising: apillar of silicon as a vertical extension of a substrate; an implanteddrain region located in a top region of the pillar; a pair of sourceregions implanted in the substrate, wherein the pair of source regionsare located on opposite sides of the pillar; first and second floatinggates located on opposite sides of the pillar and insulated from thepillar by gate oxide; first and second control gates located on oppositesides of the pillar and insulated from the first and second floatinggates such that the first and second control gates are fabricated aspolysilicon rails parallel to the pillar and leaving the top region ofthe pillar substantially exposed; and a wordline parallel to and betweenthe first and second control gates.
 2. The pair of floating gatetransistors of claim 1 wherein the pair of source regions are fabricatedas parallel source implant rails generally extending under the first andsecond control gates.
 3. The pair of floating gate transistors of claim1 wherein the first and second control gates are fabricated as unshapedpolysilicon rails with tungsten cores.
 4. A pair of floating gatetransistors comprising: a pillar of silicon as a vertical extension of asubstrate; an implanted drain region located in a top region of thepillar; a pair of source regions implanted in the substrate, wherein thepair of source regions are located on opposite sides of the pillar;first and second floating gates located on opposite sides of the pillarand insulated from the pillar by gate oxide; and first and secondcontrol gates located on opposite sides of the pillar and insulated fromthe first and second floating gates, wherein the first and secondcontrol gates are fabricated as u-shaped polysilicon rails with tungstencores that are parallel to the pillar and leave the top region of thepillar substantially exposed.
 5. The pair of floating gate transistorsof claim 4 further comprising: a drain contact vertically extending fromthe top region of the pillar; and first and second source contactsvertically extending from the pair of source regions.
 6. The pair offloating gate transistors of claim 5 further comprising: a sourceconductor coupled to a top of the first and second source contacts; anda drain conductor coupled to a top of the drain contact, wherein thesource and drain conductors extend in a horizontal direction generallyperpendicular to the control gates.
 7. A non-volatile memory devicecomprising: control circuitry to manage read and write operations; anarray of memory cells to store data; and input/output circuitry coupledto the array, wherein the array comprises a plurality of silicon pillarsas vertical extensions of a substrate; implanted drain regions locatedin a top region of the pillars; source regions implanted in thesubstrate and located between the pillars; floating gates located onopposite sides of the pillars and above the source regions, wherein thefloating gates are insulated from the pillars by a gate oxide; controlgate lines located on opposite sides of the pillars and insulated fromthe floating gates, wherein the control gates have word lines of thearray located between two control gates, the control gates runningparallel to the pillars and a top portion of each control gate extendingover the floating gates and leaving exposed the top region of thepillars; a source conductor electrically coupled to the source regions;and a bit lines electrically coupled to the drain regions, wherein thesource conductor and the bit lines extend in a horizontal directiongenerally perpendicular to the control gates.